Many stand-up type vacuum cleaners have a power head unit which includes a rotating brush driven by an electric motor via an elastic belt which is typically formed of rubber or the like. An example of a conventional system is shown in FIG. 1.
As shown in FIG. 1, the power head includes a housing 10, wheels (not shown), a suction hose which enters through an outlet 14, an electric motor 20 secured to the housing by mounting screws 23, and a brush roller or beater bar 30. A pair of shafts ends 32 extend outward from the axial ends of the beater bar 30. Each shaft end 32 is supported in a mounting plate 37 secured in a mounting groove 36 via rubber mounts 35. Typically a brush roll pulley 33 is rotatably secured to a shaft end 32 which is predictably referred to as the pulley end. A motor drive 27 extends from the motor 20. An elastic belt 40 loops around the motor drive shaft 27 and the brush roll pulley 33. Typically, the brush roll pulley 33 has a larger diameter than the motor drive shaft 27 so that the brush roll or beater bar 30 rotates at a slower rate but higher torque than the motor drive shaft. A speed reduction (torque increase) ratio of about 4:1 is normal. If the belt 40 has sufficient tension it will transfer rotation of the motor shaft 27 into rotation of the brush roll pulley 33 and hence the brush roll 30 in the known manner. In such a case, the shaft 27 is a drive shaft and the shaft ends 32, beater bar 30 and pulley 33 constitute a driven shaft.
In order to provide the necessary tension an elastic or stretch type belt is typically used. Such a belt is advantageous because it is self tensioning. However, such belts also present problems. Most notably, known stretch type belts experience inelastic stretching or deformation over time as a result of age and wear. For example a typical belt in a known system can experience an increase in length of up to 0.25 inches over time. As the belt lengthens, its tension decreases. For example, it is know that the tension on the rubber belt in one conventional vacuum can decrease from 24 pounds to 12 pounds as a result of manufacturing tolerances and age.
In order to make sure that the belt will still have sufficient tension to drive the beater bar after lengthening, the initial tension of the belt is designed to be well above the tension required to drive the brush roll beater bar 30. Thus, in the example given above, if 12 pounds of tension is sufficient, the initial tension (24 pounds) is twice the necessary tension. Consequently, a needlessly large unbalanced force is applied to the shafts 27 and 32. The action of this force F on the motor shaft 27 and shaft ends 32 is shown in FIG. 1. A higher belt tensioning force causes excess load on the bearings resulting in premature wear and failure of the bearings in the drive motor and brush roller. In most applications, require a ball bearing 5 must be used in the heavy load side of the motor (the side adjacent the belt 40) because of the magnitude of these belt loads. This is expensive and life limiting. On the opposite side, a sleeve bearing 7 is used. Again, under higher loads, edge loading occurs and bearing life is limited.
Considering first the bearings in the motor, the ball bearing 5, as mentioned above, is necessary to react belt loads. The sleeve bearing 7 experiences high edge loads and often fails. Also, in the case of plastic sleeve bearings, the bearings seize because of thermo expansion due to frictional heat generated by high loads. Similarly, the shaft ends 32 of the beater bar or brush roll 30 are typically mounted in sleeve bearings 7 or expensive ball bearings. If sleeve bearings are used, they experience high edge wear because of the unbalanced belt tension force. Thus, it can be seen that there is need for an improved bearing and bearing support for a belt driven system of this type.
This application relates, in part, to a belt driven system in which plastic sleeve bearings can be used instead of expensive ball bearings. The principal limitation in a plastic sleeve bearing's performance is the so-called PV limit. For instance, high ledge loading causes a plastic sleeve bearing to reach its PV limit. PV is the product of load or pressure (P) and sliding velocity (V). A plastic bearing subjected to increasing PV loading will eventually reach a point of failure known as the PV limit. The failure point is usually manifested by an abrupt increase in the wear rate of the bearing material.
As long as the mechanical strength of the bearing material is not exceeded, the temperature of the bearing surface is generally the most important factor in determining PV limit. Therefore, anything that affects surface temperature-- coefficient of friction, thermal conductivity, lubrication, ambient temperature, running clearance, hardness and surface finish of mating materials--will also affect the PV limit of the bearing.
Thus, the first step in selecting and evaluating a sleeve bearing is determining the PV limit required by the intended application. It is usually prudent to allow a generous safety margin in determining PV limits, because real operating conditions often are more rigorous than experimental conditions.
Determining the PV requirements of any application is a three step process. First, the static loading per unit area (P) that the bearing must withstand in operation must be determined. For journal bearing configurations the calculation is as follows: EQU P=W/(d.times.b)
where: EQU P=pressure, psi (kg/cm.sup.2)
W=static load, lb (kg) PA0 d=bearing surface ID, in. (cm) PA0 b =bearing length, in. (cm) PA0 V=surface velocity, in/min (cm/min) PA0 N=speed of rotation, rpm of cycles/min PA0 d=bearing surface ID, in. (cm)
Pressure (P) should not exceed certain maximum values at room temperature. These can be derived from a table of allowable static bearing pressure for most known materials. Next, the velocity (V) of the bearing relative to the mating surface must be calculated. For a journal bearing experiencing continuous rotation, as opposed to oscillatory motion, velocity is calculated as follows: EQU V=(dN)
where:
Finally, calculate PV as follows: EQU PV (psi-ft/min)=P (psi).times.V (in/min) 12
or, in metric units: EQU PV (kg/cm.sup.2 -m/sec)=P (kg/cm.sup.2).times.V (cm/min)/6000
The PV limits of unlubricated bearing materials are generally available from the manufacturer of the material or from technical literature. Since PV limits for any material vary with different combinations of pressure and velocity as well as with other test conditions, it is prudent to consult the manufacturer for detailed information.
One material which is particularly well suited to bearing applications is the polyimide thermoset material sold by Dupont under the trademark VESPEL.TM.. Properly lubricated VESPEL.TM. parts can withstand approximately 1 million psi-ft/min.
This application further relates to beam mounted support for the components of a vacuum head power unit. It is believed that the concept of a beam mounted support has not, to date, been applied to vacuum power heads. It is also believed that the most advanced work in the field of the deflecting beam supports is that of the present inventor. For instance, the present inventor's European Patent Application (Publication No. 0343620) describes bearings having beam mounted bearing pads and methods of making the same. The specification of this published application is incorporated herein by reference. In this case, the bearings are supported by deflecting beam support structures to assist in the formation of a hydrodynamic wedge between a bearing pad and a rotating shaft.
Other patents have disclosed flexible support structures for supporting hydrodynamic bearing pads. For instance, U.S. Pat. No. 3,107,955 to Trumpler discloses a bearing having beam mounted bearing pads that move with a pivoting or swing type motion about a center located in front of the pad surface. The beam support is based only on a two dimensional model of pad deflection.
U.S. Pat. No. 4,496,251 to Ide, the present inventor, discloses a bearing pad mounted on web-like ligaments to deflect so that a wedge shaped film of lubricant is formed between the relatively moving parts.
U.S. Pat. No. 4,676,668, also to Ide, discloses a bearing construction which includes a plurality of discrete bearing pads supported in a carrier member. Each bearing pad includes a pad portion and a beam-like support structure for supporting the pads as desired.
As mentioned above, the support structures described in these patents has not heretofore been applied to the power head of a vacuum cleaner.